Objective Most US adults are insufficiently active. One strategy individuals could use to increase physical activity is to exercise mindfully (i.e., while paying attention to present-moment experiences with acceptance. A mindfulness-based intervention for exercise can be delivered via an audio recording, which is advantageous in regard to time demands, cost, and dissemination potential. The aims of this parallel two-arm pilot randomized controlled trial were to assess the feasibility and acceptability of an audio-recorded mindfulness-based intervention and to assess whether the intervention resulted in a clinically meaningful difference in physical activity compared with the control condition. Methods Physically underactive adults (N = 50) were randomized to a mindfulness intervention condition in which they were instructed to exercise while listening to an audio-recorded mindfulness-intervention or an active control group in which they were instructed to exercise while using a heart rate monitor. Participants completed a 30-minute moderate intensity treadmill exercise bout during a baseline in-laboratory session in the manner in which they were randomized (i.e., mindfulness recording versus using a heart rate monitor) and instructed to exercise in this manner for the next week. At follow-up, acceptability was measured by self-report, feasibility by frequency of intervention use, and physical activity using both self-reported physical activity recall and an accelerometer. Results The audio-recorded mindfulness-based physical activity intervention was rated as acceptable and feasible to use. Compared with the control group, the intervention also resulted in clinically meaningful differences in self-reported moderate-to-vigorous physical activity minutes (mean difference = 67.16 minutes) and accelerometer-measured minutes (mean difference = 35.48 minutes) during a 1-week follow-up. Conclusions The audio-recorded mindfulness-based physical activity intervention is a promising approach to increasing physical activity with good dissemination potential.